That means even if carbon emissions dropped to zero tomorrow, we'd still be watching human-driven climate change play out for centuries. And, as we all know, emissions aren't going to stop tomorrow. So the key thing now, Schmidt said, is slowing climate change down enough to make sure we can adapt to it as painlessly as possible.

This is what the Earth could look like within 100 years if we do, barring huge leaps in renewable energy or carbon-capture technology.

Let's assume we land somewhere between those two targets. At the end of this century, we're already looking at a world that is, on average 3 degrees or so Fahrenheit above where we are now.

But average surface temperature alone doesn't full capture climate change. Temperature anomalies — or how much the temperature of a given area is deviating from what would be "normal" in that region — are going to swing wildly.

For example, the temperature in the Arctic Circle last winter soared above freezing for one day. It was still cold for Florida, but extraordinarily hot for the arctic. That's abnormal, and it's going to start happening a lot more.

That means years like this one, which set a record for lowest ever sea-ice extent, are going to become common. Summers in Greenland could become ice-free by 2050.

Even 2015 was nothing compared to 2012, when 90% of the Greenland Ice Sheet melted in the summer. But given the damage already done, we could see this kind of extreme melt every 6 years by the late 2000s.

But in our best-case scenarios, oceans are on track to rise by 2 to 3 feet by 2100. Even sea level rise below 3 feet could displace up to 4 million people.

Not only will oceans have less ice at the poles, but they will continue to acidify in the tropics. Oceans absorb about a third of all carbon dioxide in the atmosphere, which causes them to warm and become more acidic.

If climate change continues unabated, nearly all coral reef habitats could be devastated. Under our best case scenario, half of all tropical coral reefs are still threatened.

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But the oceans aren't the only place heating up. Even if we curb emissions, summers in the tropics could increase their extreme heat days by half after 2050. Farther north, 10% to 20% of the days in a year will be hotter.

But compare that to the business-as-usual scenario, where the tropics will stay at unusually hot temperatures all summer long. In the temperate zones, 30% or more of the days will be what is now unusual.

Even a little bit of warming will strain water resources. In a 2013 paper, scientists used models to estimate that the world could see more severe droughts more frequently — about a 10% increase. If unchecked, climate change could cause severe drought across 40% of all land, double what it is today.

And then there's the weather. If the extreme el Nino event of 2015-2016 was any indication, we're in for much more drastic natural disasters. More extreme storm surges, wildfires, and heat waves are all on the menu for 2070 and beyond.

Right now, humanity is standing on a precipice. We can ignore the warning signs, and pollute ourselves into what Schmidt envisions as a "vastly different planet" — roughly as different as our current climate is from the last ice age.

Reuters

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Or we can innovate solutions. Many of the scenarios laid out here assume we're reaching negative emissions by 2100 — that is, absorbing more than we're emitting through carbon-capture technology.

Smoke rises from the chimneys of a power plant in Shanghai December 5, 2009.
Reuters/Aly Song